1. Field of the Invention
The present invention relates to an operational amplifier used in a switched capacitor filter or the like.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a conventional operational amplifier used in a switched capacitor filter. The operational amplifier comprises a differential circuit 11 and a source ground circuit 12 connected to the differential circuit 11. The differential circuit 11 comprises P-channel MOS transistors (referred to as PMOS transistors hereinafter) 13 and 14 serving as an input transistor pair, a PMOS transistor 15 serving as a constant current supply, and N-channel MOS transistors (referred to as NMOS transistors hereinafter) 16 and 17 serving as a current mirror. The source ground circuit 12 comprises a phase-compensating capacitor 18, a resistor 19, a PMOS transistor 20 serving as a constant current supply, and an NMOS transistor 21. A high input impedance can be obtained in the above operational amplifier since the MOS transistors 13 and 14 are used as the input transistor pair. However, this operational amplifier has the following drawbacks. Since a MOS transistor has 1/f noise or the like, input conversion noise is increased. In addition, the MOS transistor has a low cutoff frequency f.sub.T, the operational amplifier cannot easily have wideband frequency characteristics. On the other hand, an operational amplifier using bipolar transistors as input transistors can have wideband frequency characteristics with low noise. However, since the bipolar transistor has a low input impedance, a drawback is posed that the bipolar transistor cannot be preferably applied to a circuit such as a switched capacitor filter (SCF).
FIG. 2 is a circuit diagram showing a conventional operational amplifier using P-channel junction FETs (referred to as JFETs). This amplifier has P-channel JFETs 22 and 23 as an input transistor pair. In order to solve the above drawbacks, JFETs having low noise and a high input impedance may be used as the input transistors of an operational amplifier. However, the operational amplifier has the following drawbacks. First, since depletion type JFETs are used as the input transistors in general, when a potential half of a power supply voltage is set as an input operating point, the transistors in the operational amplifier cannot be normally biased because the potential at the input operating point is excessively low. More specifically, this operational amplifier cannot be operated when a low power supply voltage is used. Second, when JFETs are used as an input transistor pair, noise can be reliably reduced. However, since other transistors constituting the operational amplifier are of MOS transistors, the operational amplifier cannot easily have wideband frequency characteristics. Due to the above drawbacks, the operational amplifier using JFETs as an input transistor pair cannot be easily realized. Therefore, there is only the following prior art in which an input transistor pair constituted by JFETs are used in a comparator.
FIG. 3 is a circuit diagram showing the arrangement of a conventional comparator using JFETs as an input transistor pair. This comparator is disclosed in the reference, "A Compatible CMOS-JFET Pulse Density Modulator for Interpolative High-Resolution A/D conversion"; Ulrich Roettcher et al. (IEEE J. of solid-state circuits, Vol.sc-21, No. 3, June 1986, pp 446-452). In this comparator, JFETs 31 and 32 are used as an input transistor pair. Although a detailed description of other circuit arrangements is omitted, the comparator is different from an operational amplifier in the following points. First, since an output signal need not be fed back to an input side, the transistors of the comparator can be easily biased. Second, since the output signal is not fed back to the input side, the phase of the output signal need not be compensated, and the operational amplifier can easily have wideband frequency characteristics. In this manner, the JFETs can be easily used as the input transistor pair in the comparator. However, an operational amplifier cannot be easily formed by the same technique used in the above comparator.